In plastic sheet/film extrusion, a thin film of molten polymer or melt is extruded through a die between two simultaneously fed substrates and then the layered sheet is pulled down by a nip roll assembly. As shown in FIG. 1, a typical nip roll assembly comprises a chill roll, or CI drum, and a rubber-sleeved chill roll that are aligned so as to compress the layered sheet between the CI drum and the rubber-sleeved chill roll. A back-up cooling roll is positioned to maintain the pressure from the rubber-sleeved chill roll onto the CI drum to maximize adhesion between the melt and substrates.
The substrates, now coated with the melt, gets cooled by the CI drum, which has a larger outside diameter than the other rolls. The larger outside diameter of the CI drum allows the CI drum to increase the cooling time since it has a larger area of contact with the film than any of the other rolls. After the film exits the CI drum, it is cooled and ready to go on to the next lamination stages.
Rubber sleeves are required on the smaller diameter chill roll when pulling the film due to the film's friction characteristics, and to maintain an even, flat profile. However, if the rubber gets overheated, it will eventually fail. For example, the molten polymer can easily adhere to an overheated rubber and cause a buildup on the rubber. In extreme conditions, the rubber may crack or even break from the chill roll. On the other hand, if the chill roll and CI drum temperatures are too low, adhesion between substrates and melt may be ineffective due to excessively rapid quenching of the melt. So proper temperature control is key for processing an acceptable film.
The rubber sleeves may be composed of a variety of different materials and may have different surface finishes depending on the intended use. Each sleeve material has a unique temperature range for optimum performance. Because of this, whenever there is a job change on a production line and the new job requires a rubber sleeve with a different profile, the rubber sleeves on the chill rolls need to be exchanged. Further, normal wear and tear during an extended production run can cause the rubber sleeve to fail and require replacement.
In the prior art methods, in order to do a rubber sleeve replacement the entire rubber-sleeved chill roll needs to be removed from the machine and then be replaced with a new roll. The removed rubber-sleeved chill roll can be repaired on a lathe until its surface is even and flat again, though there is a limit to how many times it can be repaired. Elasticity decreases as rubber thickness decreases, failures can happen if significant torque or shear is applied to a rubber that is too thin. On the other hand, if the rubber is too thick, it won't dissipate heat too well and cause an over heat, so minimum and maximum outside diameters must be carefully determined. This can be a time-consuming process, which may be further complicated if the rubber-sleeved chill roll is not easily accessible.